Disc drives, including magnetic disc drives, optical disc drives and magnet-optical disc drives, are widely used for storing information. A typical disc drive has one or more discs for storing information in a plurality of concentric circular tracks. This information is written to and read from the discs using read/write heads mounted on actuator arms which are moved from track to track across surfaces of the discs by an actuator mechanism. The discs are mounted on a spindle which is turned by a spindle motor to pass the surfaces of the discs under the read/write heads. The spindle motor generally includes a shaft fixed to a baseplate and a hub, to which the spindle is attached, having a sleeve into which the shaft is inserted. Permanent magnets attached to the hub interact with a stator winding on the baseplate to rotate the hub relative to the shaft. One or more bearings between the hub and the shaft facilitate rotation of the hub.
The spindle motor also typically includes an exclusion seal to seal interfacial spaces between the hub and the shaft. This is necessary, because lubricating fluids or greases used in the bearings tend to give off aerosols or vaporous components that migrate or diffuse out of the spindle motor and into a disc chamber in which the discs are maintained. This vapor often transports other particles, such as material abraded from the bearings or other components of the spindle motor, into the disc chamber. These vapors and particles deposit on the read/write heads and the surfaces of the discs, causing damage to the discs and the read/write heads as they pass over the discs. Thus, the migration of these contaminants into the disc chamber must be prevented.
To prevent the migration of these contaminants into the disc chamber, the latest generation of spindle motors utilize a ferrofluidic seal between the shaft and the hub. Ferrofluidic seals are described in, for example, U.S. Pat. No. 5,473,484, which is incorporated herein by reference. A typical ferrofluidic seal consists of a ferrofluid, an axially polarized annular magnet and two magnetically permeable annular pole pieces attached to opposing faces of the magnet. The ferrofluid is conventionally composed of a suspension of magnetically permeable particles suspended in a fluid carrier. Generally, the magnet and the pole pieces are fixed to the hub and extend close to but do not touch the shaft. Magnetic flux generated by the magnet passes through the pole pieces and the shaft, which is also magnetically permeable, to magnetically hold the ferrofluid in gaps between the pole pieces and the shaft, thereby forming a seal.
As noted above, current ferrofluidic seals use a rotating design in which the magnet and pole pieces are affixed to the hub, and the ferrofluid seals with a fixed shaft. Although this design has worked well for conventional spindle motors, the latest generation of motors rotate at high speeds, often exceeding 13,000 revolutions per minute (rpm). The centrifugal forces developed at such high speeds often exceed the ability of the ferrofluidic seal's magnetic flux to hold the ferrofluid against the shaft due to the velocity gradient across the ferrofluid, resulting in the failure of the ferrofluidic seal to maintain a hermetic seal.
Accordingly, there is a need for a ferrofluidic seal that seals an outer surface of a shaft to an inner surface of a hub disposed about the shaft. It is desirable that the ferrofluidic seal provide a structure that is reliable at high rotational speeds. There is also a need for a method of forming such a ferrofluidic seal that does not increase either manufacturing time or costs for assembling a spindle motor in which the seal is used.
The present invention provides a solution to these and other problems, and offers other advantages over the prior art.